1. Field of the Invention
The present invention is related to solid state storage and particularly to solid state storage cells with a phase change material memory element.
2. Background Description
Solid state, phase change materials that are chalcogen (Group VI elements such as sulfur (S), selenium (Se) and tellurium (Te)) alloys with at least one of germanium (Ge), arsenic (As), silicon (Si), and antimony (Sb) are known as chalcogenides and are well known. Chalcogenides exist in at least two different classifiable solid states or phases. The most extreme two states can be classified simply as amorphous and crystalline states with other less easily discernable states ranging between those two states. The amorphous state has a disordered atomic structure and the crystalline state generally is polycrystalline. Each phase has very different electrical properties. In its amorphous state, the material behaves as an insulator, i.e., an open circuit; in its crystalline state, the same material behaves resistively. The resistivity of these materials varies in between amorphous and crystalline states by as much as 6 orders of magnitude.
In particular, when heat is applied to some phase change chalcogenides, the material switches phases from one (e.g., amorphous phase) state to a second (e.g., crystalline phase) state. The transitions between these states is selectively reversible with heat, i.e., the phase change material may be set/reset. As with anything that has two or more discernable and selectable states, each of the 2 stable states can be designated as a logic one and the other a logic zero. Thus, phase change material has found use in storage devices and particularly, for non-volatile storage, e.g., as a memory cell storage media. In addition, multiple bit memory elements have been made using the intermediate states inherent in the variation in resistivity between amorphous and crystalline.
Typically, controlled heat must be precisely provided to the phase change storage media to effect reversible transitions between amorphous and crystalline states and back. Normally, such heat is provided using resistive heating. Unfortunately, a relatively large current is needed for each memory element to heat the phase change material. In particular, resetting the phase change material may require heating the crystalline material to its melting point, generally above 600° C. Thus, it maybe difficult on one hand to provide sufficient localized heat to raise the crystalline phase change material to its melting point and, on the other hand, to avoid accidentally heating other adjacent cells to the phase change set point and inadvertently setting adjacent cells.
One prior approach to localize heating in phase change switching is to minimize the phase change material contact area. Unfortunately, reducing the contact area normally increases cost, which is inversely proportional to the size of the contact. In particular, attaining a contact size below the minimum photolithographic image size for a particular technology complicates the process significantly and, correspondingly, increases cost. Further, normally, reducing contact size, reduces the heat delivery capability and increases resistance/reduces current delivered to the phase change material; all of which interferes with setting/resetting the material rather than enhancing it.
Thus, there is a need for improved and very localized or focused heat delivery to phase change material in memory cells.